Escherichia coli is a normal inhabitant of the lower gastrointestinal tract of animals, including humans, and is commonly found in fecal matter and raw sewage. Contamination of food with E. coli, often due to unsanitary conditions, can result in food poisoning, with clinical signs ranging from mild nausea and diarrhea, to severe illness and even death.
Fortunately, food poisoning with most strains of E. coli causes only mild illness. However, in recent years, a number of deaths have occurred in the United States and elsewhere due to outbreaks of illness following ingestion with food bearing E. coli bacteria. The causative organism of several of these outbreaks has been identified as a particular pathogenic strain of E. coli known
E. coli O157:H7, unlike ordinary commensal E. coli is an enterohemorrhagic bacterium and causes enterohemorrhagic colitis, hemolytic uremic syndrome, and mesenteric adenitis. The O157:H7 serotype is the most frequent enterohemorrhagic E. coli, and the most common cause of these life threatening illnesses.
E. coli O157:H7 infections can arise from a variety of sources but most commonly by the ingestion of contaminated foods or beverages. These foods, most commonly meat and dairy products, but also vegetables and fruits, such as berries, which are contaminated with E. coli O157:H7 are indistinguishable on visual or other sensory inspection from food which is not contaminated or which is contaminated with other strains of E. coli. Present methods for identifying E. coli O157:H7 are time consuming, expensive, and non-specific. Thus, these methods are not optimal for routine testing of food for the detection of the O157:H7 strain.
One such currently available method involves a multi-day test whereby a suspected fecal or food sample is placed in an enrichment medium to select for gram negative bacteria. The sample is then incubated on filter paper on which colonies of E. coli O157:H7 will grow and produce toxins called verotoxins. The toxins are trapped in the filter paper and are identified using antibodies which specifically bind to the verotoxins.
Another method, disclosed in U.S. Pat. No. 5,354,661 (1994), utilizes a monoclonal antibody in an ELISA test to detect serotype O157:H7 in a sample. This method, however, has several disadvantages. For one thing, a presumptive diagnosis of serotype O157:H7 is obtained after about 20 hours, with a definitive diagnosis within an additional two days. Second, the antibody of the '661 patent is not specific for serotype O157:H7, but rather detects one other E. coli serotype, thereby possibly resulting in false positive tests.
U.S. Patent No. 5,475,098 (1995) discloses the detection of enterohemorrhagic E. coli, including serotype O157:H7, by the presence of a signature DNA sequence which is common to the group. False positive results are associated with this method as enterohemorrhagic E. coli other than O157:H7 are detected.
A serious need exists for a method of detection of O157:H7 which is inexpensive, rapid, and specific for this serotype. It is only by such a method that the risk of disease due to foods contaminated with O157:H7, or that serotype O157:H7 itself, can ever be eliminated.